Devices for converting direct current to alternating current are well known and have been used, for instance, at the receiving end of long distance transmission lines. Normally, these devices employ transformers that are massive relative to the power levels involved which implies high cost and precludes their use in many applications, notably downhole use in oil field operations.
It has been determined that the mass of a transformer is approximately directly proportional to its power handling capability and inversely proportional to the frequency of operation. Since most systems used in the United States operate at relatively low frequencies (e.g. approximately 60 cycles per second), it is obvious that commonly used high power/low frequency transformers are massive. Accordingly, by performing the necessary transformations at a relatively high frequency (e.g. equal to or greater than 10,000 cycles per second), the mass of the transformer may be lowered.
To accomplish these high frequency transformations, the present invention makes use of the trigonometric identity, sin (a) sin (b)=1/2 cos (a-b)-1/2 cos (a+b). This identity implies that the multiplication of two sine waves results in a wave form containing the sum and the difference of the two original frequencies. As an example, if the original frequencies are 10,000 cps and 10,060 cps, the resulting wave form consists of, primarily 20,060 cps and 60 cps components. If this wave form is then rectified by a synchronous rectifier, driven in synchronism with the 60 cps component and the resulting wave form passed through a low pass filter to remove the 20,060 cps component, the result is a nearly pure 60 cps sine wave.
In accordance with the invention, power converting apparatus for changing direct current into a chosen frequency alternating current comprises a transformer having a primary winding with a direct current supplied intermediate its ends and a secondary winding with a ground being mounted intermediate its ends. Means are mounted with the transformer for converting the direct current to a series of pulses in the primary winding which includes a pulse train forming an effective current with reciprocal positive and negative sine wave components for each half cycle of the chosen frequency. Gating means are mounted with the secondary of the transformer for sequentially passing the pulses in the negative sine wave component of the effective current followed by the pulses in a positive sine wave component. The gating means pass the pulses received in the secondary winding synchronously with pulses in each half cycle of the pulse train to form a chosen frequency alternating current component. A filter is mounted with the gating means for removing selected components from the gated signal while providing an output of chosen frequency alternating current.
Also, in accordance with the invention, a method of converting the direct current into chosen frequency alternating current comprises converting the direct current into a series of pulses which includes a pulse train forming an effective current with reciprocal positive and negative sine wave components for each half cycle of the chosen frequency. These pulses are supplied to the primary winding of the transformer and pulses received by the secondary winding of the transformer are then gated synchronously with the half cycles of the chosen frequency for alternately removing the negative and positive sine wave components of the effective current. The gated current is then passed through a filter to remove selected components of the received signal while providing an output of the chosen frequency alternating current.